The present invention relates to a method and apparatus for making multilayer injection-molded plastic articles such as preforms, wherein the successive molding of an inner sleeve and outer layer enables cost-effective production of multilayer preforms for pasteurizable, hot-fillable, and returnable and refillable beverage containers.
There is described in U.S. Pat. No. 4,609,516 to Krishnakumar et al. a method for forming multilayer preforms in a single injection mold cavity. In that method, successive injections of different thermoplastic materials are made into the bottom of the mold cavity. The materials flow upwardly to fill the cavity and form for example a five-layer structure across the sidewall. This five-layer structure can be made with either two materials (i.e., the first and third injected materials are the same) or three materials (i.e., the first and third injected materials are different). Both structures are in widespread commercial use for beverage and other food containers.
An example of a two-material, five-layer (2M, 5L) structure has inner, outer and core layers of virgin polyethylene terephthalate (PET), and intermediate barrier layers of ethylene vinyl alcohol (EVOH). An example of a three-material, five-layer (3M, 5L) structure has inner and outer layers of virgin PET, intermediate barrier layers of EVOH, and a core layer of recycled or post-consumer polyethylene terephthalate (PC-PET). Two reasons for the commercial success of these containers are that: (1) the amount of relatively expensive barrier material (e.g., EVOH) can be minimized by providing very thin intermediate layers; and (2) the container resists delamination of the layers without the use of adhesives to bond the dissimilar materials. Also, by utilizing PC-PET in the core layer, the cost of each container can be reduced without a significant change in performance.
Although the above five-layer, and other three-layer (see for example U.S. Pat. No. 4,923,723) structures work well for a variety of containers, as additional high-performance and expensive materials become available there is an on-going need for processes which enable close control over the amount of materials used in a given container structure. For example, polyethylene naphthalate (PEN) is a desirable polyester for use in blow-molded containers. PEN has an oxygen barrier capability about five times that of PET, and a higher heat stability temperaturexe2x80x94about 250xc2x0 F. (120xc2x0 C.) for PEN, compared to about 175xc2x0 F. (80xc2x0 C.) for PET. These properties make PEN useful for the storage of oxygen-sensitive products (e.g., food, cosmetics, and pharmaceuticals), and/or for use in containers subject to high temperatures (e.g., refill or hot-fill containers). However, PEN is substantially more expensive than PET and has different processing requirements: Thus, at present the commercial use of PEN is limited.
Another high-temperature application is pasteurizationxe2x80x94a pasteurizable container is filled and sealed at room temperature, and then exposed to an elevated temperature bath for about ten minutes or longer. The pasteurization process initially imposes high temperatures and positive internal pressures, followed by a cooling process which creates a vacuum in the container. Throughout these procedures, the sealed container must resist deformation so as to remain acceptable in appearance, within a designated volume tolerance, and without leakage. In particular, the threaded neck finish must resist deformation which would prevent a complete seal.
A number of methods have been proposed for strengthening the neck finish. One approach is to add an additional manufacturing step whereby the neck finish, of the preform or container, is exposed to a heating element and thermally crystallized. However, this creates several problems. During crystallization, the polymer density increases, which produces a volume decrease; therefore, in order to obtain a desired neck finish dimension, the as-molded dimension must be larger than the final (crystallized) dimension. It is thus difficult to achieve close dimensional tolerances and, in general, the variability of the critical neck finish dimensions after crystallization are approximately twice that prior to crystallization. Another detriment is the increased cost of the additional processing step, as it requires both time and the application of energy (heat). The cost of producing a container is very important because of competitive pressures and is tightly controlled.
An alternative method of strengthening the neck finish is to crystallize select portions thereof, such as the top sealing surface and flange. Again, this requires an additional heating step. Another alternative is to use a high Tg material in one or more layers of the neck finish. This also involves more complex injection molding procedures and apparatus.
Thus, it would be desirable to provide an injection-molded article such as a preform which incorporates certain high-performance materials, and a commercially acceptable method of manufacturing the same.
The present invention is directed to a method and apparatus for making a multilayer injection-molded plastic article, such as a preform, which is both cost-effective and enables control over the amounts of materials used in the various layers and/or portions of the article.
According to a method/embodiment of the invention, an inner sleeve is molded on a first core positioned in a first mold cavity. The inner sleeve is only partially cooled before being transferred while still at an elevated temperature to a second mold cavity where an outer layer is molded over the inner sleeve. By providing the inner sleeve in the second mold cavity at the elevated temperature, bonding between the inner sleeve and outer layer is enabled during the second molding step, such that layer separation is avoided in the final molded article. The inner sleeve may comprise a full-length inner sleeve, extending substantially the full length of the article, or alternatively may comprise only an upper portion of the article, in which case the outer layer comprises a lower portion of the article and there is some intermediate portion in which the outer layer is bonded to the inner sleeve.
In one embodiment, a first thermoplastic material is used to make an inner sleeve which comprises a neck finish portion of the preform. The first thermoplastic material is preferably a thermal resistant material having a relatively high Tg, and/or forms a crystallized neck finish during the first molding step. In contrast, a lower body portion of the preform is made of a second thermoplastic material having a relatively lower thermal resistance and/or lower crystallization rate compared to the first material, and forms a substantially amorphous body-forming portion of the preform. In one example, by achieving crystallization in the neck finish during the first molding step, the initial and finish dimensions are the same so that the dimensional variations caused by the prior art post-molding crystallization step (and the expense thereof) are eliminated. Also, a higher average level of crystallization can be achieved in the finish, by utilizing the higher melt temperatures and/or elevated pressures of the molding process.
In another embodiment, a full-length body sleeve is provided made of a high-performance thermoplastic resin, such as PEN homopolymer, copolymer or blend. The PEN inner sleeve provides enhanced thermal stability and reduced flavor absorption, both of which are useful in refill applications. The amount of PEN used is minimized by this process which enables production of a very thin inner sleeve layer, compared to a relatively thick outer layer (made of one or more lower-performance resins).
Another aspect of the invention is an apparatus for the cost-effective manufacture of such preforms. The apparatus includes at least one set of first and second molding cavities, the first mold cavity being adapted to form the inner sleeve and the second mold cavity adapted to form the outer layer. A transfer mechanism includes at least one set of first and second cores, wherein the cores are successively positionable in the first and second molding cavities. In one cycle, a first core is positioned in a first mold cavity while a first inner sleeve is molded on the first core, while a second core, carrying a previously-molded second inner sleeve, is positioned in a second mold cavity, for molding a second outer layer over the second inner sleeve. By simultaneously molding in two sets of cavities, an efficient process is provided. By molding different portions/layers of the articles separately in different cavities, different temperatures and/or pressures may be used to obtain different molding conditions and thus different properties in the different portions/layers. For example, it is possible to mold the crystallized neck finish portion in a first cavity, while molding a substantially amorphous outer layer in the second cavity.
The resulting injection-molded articles, and/or expanded injection-molded articles, may thus have a layer structure which is not obtainable with prior processes.
The following chart provides temperature/time/pressure ranges for certain preferred embodiments, which are described in greater detail in the following sections:
a) for an inner sleeve of PEN polymer material and an outer layer of PET polymer material
b) for an inner sleeve of crystallized polyester material and an outer layer of PET polymer material
The present invention will: be more particularly set forth in the following detailed description and accompanying drawings.